This invention relates to reliability-enhancing layers for vertical cavity surface emitting lasers, and methods of making the same.
A vertical cavity surface emitting laser (VCSEL) is a laser device formed from an optically active semiconductor layer (e.g., AlInGaAs or InGaAsP) that is sandwiched between a pair of highly reflective mirror stacks, which may be formed from layers of metallic material, dielectric material or epitaxially-grown semiconductor material. Typically, one of the mirror stacks is made less reflective than the other so that a portion of the coherent light that builds in a resonating cavity formed between the mirror stacks may be emitted from the device. Typically, a VCSEL emits laser light from the top or bottom surface of the resonating cavity with a relatively small beam divergence. VCSELs may be arranged in singlets, one-dimensional or two-dimensional arrays, tested on wafer, and incorporated easily into an optical transceiver module and coupled to a fiber optic cable.
In general, a VCSEL may be characterized as a gain-guided VCSEL or an index-guided VCSEL. An implant VCSEL is the most common commercially available gain-guided VCSEL. An implant VCSEL includes one or more high resistance implant regions for current confinement and parasitic reduction. An oxide VCSEL, on the other hand, is the most common index-guided (laterally and vertically) VCSEL. An oxide VCSEL includes oxide layers (and possibly implant regions) for both current and optical confinement.
VCSELs and VCSEL arrays have been successfully developed for single-mode operation and multi-mode operation at a variety of different wavelengths (e.g., 650 nm, 850 nm, 980 nm, 1300 nm and 1550 nm). The commercial success of VCSEL technology, however, will depend in large part upon development of VCSEL structures that are characterized by high performance and high reliability.
Techniques have been proposed for improving the performance and reliability of a wide variety of different semiconductor laser devices, including VCSELs and edge-emitting lasers.
For example, U.S. Pat. No. 5,838,705 discloses VCSEL devices (i.e., a non-planar ridge VCSEL and a planar implant VCSEL) that include one or more defect inhibition layers that are positioned in a respective one of two cladding regions that are formed on opposite sides of an active area. According to the ""705 patent, the defect inhibition layers may be disposed anywhere outside of the active area. However, the only preferred locations for the defect inhibition layers are in close proximity and on either side of the active area to provide a barrier that does not allow defects formed outside of the active area to pass through and into the active area. The defect inhibition layers are formed from an indium-containing material that induces strain in the VCSEL device. The strain is believed to either prohibit movement of defects to the active area or attract and, subsequently, trap defects in the defect inhibition layers.
U.S. Pat. No. 4,984,242 discloses a GaAs/AlGaAs edge-emitting laser that includes at least one cladding layer that includes indium. According to the ""242 patent, the indium creates a local strain field that is sufficient to reduce and effectively stop defect migration through the cladding layer. The indium-containing strain layer may be spaced apart from the active region or may be positioned adjacent to the active region. Indium-containing layers may be added to the active region barrier layers to improve the performance of the edge-emitting laser. In one embodiment, a uniform doping of indium is provided throughout the edge-emitting laser heterostructure to impede the growth and migration of defects in the crystal lattice. Another embodiment includes indium in a cap layer to reduce the surface work function and, thereby, reduce the contact resistance of an overlying metallization layer. The ""242 patent does not teach or suggest the use of indium in a VCSEL, nor does it teach or suggest how indium might be translated to a VCSEL structure.
The invention features reliability-enhancing layers that perform specific functions at one or more critical locations within a VCSEL structure to reduce or prevent defect formation and migration that otherwise might degrade VCSEL performance, for example, by increasing optical absorption in the mirror stacks or by degrading the electro-optic properties of the active region. In particular, the reliability-enhancing layers are configured to perform one or more of the following functions within the VCSEL structure: gettering (i.e., removing defects or impurities from critical regions), strain balancing (i.e., compensating the lattice mismatch in the structure to minimize strain), and defect suppression (i.e., block/reduce defects formation/migration during growth, processing or device operations). By strategically positioning one or more appropriately configured reliability-enhancing layers with respect to an identified defect source, the invention enables VCSEL structures to be modified in a way that enhances the reliability and performance of is VCSEL devices.
In one aspect, the invention features a vertical cavity surface emitting laser (VCSEL) that includes a first mirror stack, a second mirror stack, and a cavity region that is disposed between the first mirror stack and the second mirror stack and includes an active region. The VCSEL also includes a defect source and a reliability-enhancing layer positioned with respect to the defect source to reduce defect-induced degradation of one or more VCSEL regions.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The reliability-enhancing layer may be positioned between the defect source and the cavity region, within the defect source, or in close proximity to the defect source (above or below, or both).
As used herein, the term xe2x80x9ccavity regionxe2x80x9d refers to the VCSEL structure that includes the active region and the spacer layers.
The reliability-enhancing layer may include one or more of the following elements: indium, boron, phosphorus, antimony, and nitrogen. The reliability-enhancing layer may be lattice-matched to surrounding layers. Alternatively, the reliability-enhancing layer may include one or more strained layers. The reliability-enhancing layer may include a superlattice, which may be tensile strained, compressive strained or strain compensated. The reliability-enhancing layers may be separated by non-reliability-enhancing layers.
The defect source may include one or more of the following: an oxidized portion of the VCSEL, an implant region of the VCSEL, an exposed region of the VCSEL, one or more dielectric layers, a doped region of the VCSEL, and the substrate.
The reliability-enhancing layer may be configured to balance strain created by the defect source. For example, the defect source may include an oxide region that induces a compressive strain field, and the reliability-enhancing layer may be positioned within the compressive strain field and may be characterized by tensile strain that substantially balances the compressive strain field.
In some embodiments, the defect source creates a concentration gradient that induces defect migration. In these embodiments, the reliability-enhancing layer may be configured to reduce the induced defect migration. For example, the defect source may be characterized by a relatively high group V vacancy concentration, in which case, the reliability-enhancing layer preferably is characterized by a lower diffusion rate of vacancy defects.
In another aspect, the invention features a method of manufacturing a VCSEL. In accordance with this method, a first mirror stack is formed, a second mirror stack is formed, and a cavity region having an active region is formed therebetween. A defect source is formed, and a reliability-enhancing layer is positioned with respect to the defect source to reduce defect-induced degradation of one or more VCSEL regions.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.